Rick Perry has become the latest Republican to drop out of the race for the White House, leaving just four candidates ahead of this weekend's South Carolina primary.

Announcing it was time to beat a "strategic retreat", Mr Perry endorsed Newt Gingrich, hailing him as a "conservative visionary" who could "transform the country".

But Mr Gingrich was facing fresh problems of his own after his second wife appeared on ABC America to say he had asked her for an open marriage after she found out that he was having an affair with the woman who went on to become his third wife.

Meanwhile Republican bosses declared that Rick Santorum had won the Iowa caucuses, upending frontrunner Mitt Romney who was originally shown to have won the first-in-the-US presidential nomination vote two weeks ago.

Mr Perry's campaign had long been flagging, but with Mr Gingrich closing on Mr Romney ahead of Saturday's vote in South Carolina, his departure could leave a big imprint on the race to find the nominee to face president Barack Obama in November.

As a Texan, I've never shied away from a fight, particularly when I considered the cause to be righteous, but... I know when it's time to make a strategic retreat.

Texas governor Rick Perry

"I have come to the conclusion that there is no viable path forward for me in this 2012 campaign. Therefore today I am suspending my campaign and endorsing Newt Gingrich for president of the United States," Mr Perry told reporters in North Charleston, South Carolina.

"As a Texan, I've never shied away from a fight, particularly when I considered the cause to be righteous, but... I know when it's time to make a strategic retreat."

There were huge expectations when the popular Texas governor entered the race, but his support dropped rapidly after bad debate performances where he displayed a weak grasp of policy, and made a series of embarrassing gaffes.

After a series of earlier stumbles, Mr Perry appeared to put the final nail in his campaign's coffin in mid-November when he forgot a crucial part of his stump speech and stammered for an agonising 53 seconds.

The states' rights champion named the departments of commerce and education as targets for elimination but could not come up with the third, the energy department.

"The third one, I can't, I'm sorry, I can't. Oops," he said.

Fatal gaffe

In his exit speech on Thursday, there was another cringe-worthy moment when Mr Perry paused after saying: "As a former Air Force pilot I don't get confused." Prepared remarks showed he intended to say something quite different.

But his final act, a qualified endorsement of Mr Gingrich, had the potential to shift momentum towards the former Republican speaker just 48 hours before the crucial primary in South Carolina.

"I believe that Newt is a conservative visionary who can transform our country. We have had our differences, which campaigns will inevitably have. Newt is not perfect, but who among us is," he said.

Mr Gingrich had trailed Mr Romney by double digits in South Carolina polls. But the latest surveys, after a strong debate performance by Mr Gingrich on Monday, showed him closing fast on the former Massachusetts governor.

Mr Gingrich has worked to position himself as the strongest conservative challenger to Mr Romney, who faces stubborn doubts about his conservative credentials and has yet to rally a majority of Republicans behind him.

At a campaign event shortly after Mr Perry's announcement, Mr Gingrich told a crowd he was "very honoured and very humbled" to have the governor's endorsement.

'Open marriage'

Much has been made of Mr Gingrich's three marriages and infidelities, but he tells voters he has sought forgiveness from God and is a changed man.

However his second wife has now gone public with details that could prove embarrassing.

Marianne Gingrich told ABC America that she learnt her marriage was over in a phone call with her husband in 1999.

"I said to him, 'Newt we've been married a long time', and he said, 'Yes, but you want me all to yourself. Callista [Callista Bisek - now Callista Gingrich] doesn't care what I do'," she said.

"He was asking to have an open marriage and I refused."

But despite the religiously conservative nature of the Republican base, there are questions whether the latest revelations will affect Mr Gingrich's chances.

Karen Martin, an original Tea Party member, says they will not change her mind, and she will do anything but vote for Mitt Romney.

"I don't know why it would change because this is not news. We've all known this all along. It has been a part of Newt's political resume for years and years and year and years so there really is no changing about it," he said.

"If you look at the polls, if they weren't concerned yesterday or last week or last month or even last year, why would they be concerned today? This is not new news."

The four remaining candidates - Mr Gingrich, Mr Romney, Mr Santorum and veteran Texas congressman Ron Paul - will battle it out in a final debate before Saturday's vote.

Free trade is the oldest argument in federal politics and the issue that literally defined the federation era but opposition exists to the TPP, courtesy of the Investor-State Dispute Resolutions clause.